a) Field of the Invention:
The present invention relates to a real image type variable magnification viewfinder optical system which is to be used with photographic cameras, video cameras and so on.
b) Description of the Prior Art:
The inverted Galilean viewfinder optical system is known well as a finder optical system which is used separately from a photographing optical system. However, this type of viewfinder optical system has a defect that it does not permit clear observation of a visual field frame or a visual field itself due to ghost and flare produced by a half mirror used for forming the visual field frame.
In contrast, the Keplerian viewfinder optical system, which is configured to permit observing a real image formed by an objective lens system, is almost free from the above-mentioned defect of the Galilean viewfinder optical system and allows clear observation of the visual field frame and the visual field.
As examples of the Keplerian viewfinder optical system having a variable magnification function, there are known two types: one which comprises an objective lens system consisting of two lens units and the other which comprises an objective lens system consisting of three lens units. As the former type of Keplerian viewfinder optical systems, there are known those disclosed by Japanese Patent Preliminary Publication Nos. Sho 61-156018, Sho 64-65519 and Hei 1-257817. Further, known as the latter type is a viewfinder optical system proposed by Japanese Patent Preliminary Publication No. Hei 1-131510.
However, the former type of viewfinder optical systems have a common defect that they have low magnification ratios though these optical systems have relatively long distances as measured from the final surfaces of the objective lens systems to intermediate image surfaces, i.e., long back focal lengths, and total lengths of the optical systems can be shortened by folding back optical paths therein with first reflecting surfaces for erecting images arranged within the distances.
On the other hand, the latter type of viewfinder optical system can easily have a variable magnification ratio of 2 or higher. When attention is paid only to a low magnification position of the latter type of viewfinder optical system, it has a short distance as measured from a first lens unit to a third lens unit and a long back focal length, and it may therefore seem that a total length of the viewfinder optical system can be shortened by folding back an optical path therein with a first reflecting surface arranged between the third lens unit and the intermediate image surface as in the case of the former type of viewfinder optical system.
However, it is practically impossible to arrange the first reflecting surface between the third lens unit and intermediate image surface since the back focal length of the latter type of viewfinder optical system at a high magnification position thereof is extremely shortened by arranging the first reflecting surface at such a location. It is therefore necessary to arrange the first reflecting surface at a location after the intermediate image surface. Consequently, the latter type of the viewfinder optical system has a defect that it cannot have a shortened back focal length.